1. Field of the Invention
The present invention relates to an optical disk device that uses a laser or other such light source to reproduce signals on an information carrier (including various kinds of information carrier, such as those used only for reproduction and those used for both recording and reproduction), and more particularly to an optical disk device having a tracking control means for controlling a light spot so that it will accurately scan a track. The present invention also relates to a loop gain setting method and a loop gain setting program with which the loop gain of tracking control is set.
2. Background Information
Digital versatile disks (hereinafter referred to as DVDs) have gained prominence in recent years as high-density optical disks that allow a large quantity of digital information to be recorded.
FIG. 5 is a schematic of the structure of a DVD-RAM, which is an example of a high-density optical disk. FIG. 5a is an overall diagram of an optical disk 506. The optical disk 506 is made up of two different doughnut-shaped regions (regions 1 and 2) separated in the radial direction of the disk. Each of these regions has a plurality of tracks. Region 2 has a phase-change film and allows the optical recording or reproduction of information (hereinafter referred to as the RAM region).
FIG. 5b is a cross section of the optical disk 506, cut radially in the RAM region. As shown in FIG. 5b, tracks which are continuous guide grooves are formed at a specific spacing on the substrate surface in the RAM region. These tracks have a pitch of about 1.6 μm. In addition, in this RAM region, convex grooves (hereinafter referred to as groove tracks) and portions sandwiched between these groove tracks (hereinafter referred to as land tracks) are both used as tracks for the recording or reproduction of information.
Meanwhile, in region 1, pits are formed in the tracks by interrupting the grooves. Region 1 is a reproduction-only region in which information is prerecorded by means of these pits (hereinafter referred to as the ROM region).
FIG. 5c is a cross section of the optical disk 506, cut radially in the ROM region. As shown in FIG. 5c, the track pitch is about 0.8 μm in the ROM region.
With a conventional optical disk device, in order to perform stable tracking control of the optical disk 506 during the reproduction or recording of information, the tracking control is performed by switching between tracking error signal detection methods for the RAM region and the ROM region (see, for example, Japanese Laid-Open Patent Application H10-124900 (paragraphs 0022 to 0046, FIGS. 1 to 5)).
A conventional optical disk device will now be described in which tracking control is performed by switching between tracking error signal detection methods in the RAM region and in the ROM region.
FIG. 6 is a block diagram of the configuration of a conventional optical disk device. In FIG. 6, an optical head 100 is made up of a light source 101, a collimator lens 102, a polarizing beam splitter 103, a quarter wavelength plate 104, an objective lens 105, a converging lens 107, a detector 108, and a tracking actuator 123.
The light source 101 is a semiconductor laser device, which outputs an optical beam onto the information side of the optical disk 506. The collimator lens 102 converts the divergent light emitted from the light source 101 into parallel light. The polarizing beam splitter 103 is an optical device that reflects all of the linear polarized light emitted from the light source 101, and transmits all of the linear polarized light perpendicular to the linear polarized light emitted from the light source 101. The quarter wavelength plate 104 is an optical device that converts the transmitted polarized light from circular polarized light to linear polarized light, or from linear polarized light into circular polarized light. The objective lens 105 converges the optical beam onto the information side of the optical disk 506. The converging lens 107 converges the optical beam transmitted by the polarizing beam splitter 103 on the detector 108. The detector 108 is a device that converts the light it receives into an electrical signal, and is split up into four detection regions. The tracking actuator 123 is a member that moves the focal point of the optical beam in the radial direction of the optical disk 506.
FIG. 7 is a plan view of the detector 108. As shown in FIG. 7, the detector 108 has four detection sub-regions A, B, C, and D. The left-right direction in the drawing is the radial direction of the optical disk 506 (hereinafter referred to as the tracking direction), while the vertical direction is the track lengthwise direction.
Preamps 109a to 109d are electrical devices that convert the output current of the four detection sub-regions A to D of the detector 108 into voltage. Adders 110a to 110d are electrical circuits that add two of the outputs of the preamps 109a to 109d and output the result. A subtracter 111 is an electrical circuit that subtracts the two output signals of the adders 110c and 110d and outputs the result. Comparators 112a and 112b are electrical circuits that digitize the outputs of the adders 110a and 110b. A phase comparator 113 compares the digitized signal outputted from the comparators 112a and 112b and outputs pulses with a time width corresponding to the phase advance or phase delay of the edge. A low pass filter 114 is an electrical circuit that smoothes the pulse signals outputted from the phase comparator 113. A switch 115 is an electrical circuit that outputs either the output signal from the low pass filter 114 or the output signal from the subtracter 111 according to a command signal from a microcomputer 119. A tracking controller 116 is a circuit that outputs a tracking control signal on the basis of the output signal from the switch 115. An A/D converter 117 is a circuit that samples the tracking control signal from the tracking controller 116 and converts it into a discrete signal. A disturbance generator 118 is a circuit that outputs a disturbance signal of a specific frequency according to a command from the microcomputer 119. An adder 120 is an electrical circuit that adds the tracking control signal from the tracking controller 116 and the disturbance signal from the disturbance generator 118 and outputs the result. A gain adjuster 121 is an electrical circuit that can set the gain to the desired value on the basis of a command signal from the microcomputer 119. A tracking driver 122 is a circuit that outputs a tracking actuator drive signal on the basis of the signal outputted from the gain adjuster 121. The tracking actuator 123 is an element that moves the objective lens 105 in the radial direction of the optical disk 506. An adder 124 is an electrical circuit that adds the two output signals of the adders 110c and 110d and outputs the result. An address regenerator 125 is a circuit that reads and outputs an address from the total amount of light obtained at the detector 108. A comparator 126 is an electrical circuit that digitizes and outputs the output signal from the switch 115. A pulse counter 127 is a circuit that counts the number of rising edges of the digitized signal outputted from the comparator 126. A memory 128 is a storage circuit for holding data. A transport motor driver 129 is a circuit that amplifies and outputs a transport motor drive signal outputted from the microcomputer 119. A transport motor 130 is an element that moves the optical head 100 in the radial direction of the optical disk 506.
The operation of a conventional optical disk device configured as above will be described through reference to FIG. 6.
The optical beam of linear polarized light emitted from the light source 101 is incident on the collimator lens 102 and converted into parallel light by the collimator lens 102. The optical beam that has been made into parallel light by the collimator lens 102 is incident on the polarizing beam splitter 103. The optical beam reflected by the polarizing beam splitter 103 is converted into circular polarized light by the quarter wavelength plate 104. The optical beam converted into circular polarized light by the quarter wavelength plate 104 is incident on the objective lens 105, and is focused on the optical disk 506. The optical beam reflected by the optical disk 506 is transmitted through the polarizing beam splitter 103 and is incident on the converging lens 107. The optical beam that was incident on the converging lens 107 is then incident on the four sub-regions A to D of the detector 108. The optical beam incident on the four sub-regions A to D of the detector 108 is converted into electrical signals for each region. The electrical signals converted for each region of the detector 108 are converted into voltage by the preamps 109a to 109d. 
The tracking control operation in the RAM region will now be described.
The output signals from the preamps 109a and 109b are added by the adder 110c. The output signals from the preamps 109c and 109d are added by the adder 110d. The output signals from the adders 110c and 110d are subtracted by the subtracter 111, which gives a tracking error signal (hereinafter referred to as TE signal) indicating the positional relation between a track and the light spot on the optical disk 506.
The above method for detecting TE signals is generally called a push-pull method. If the optical beam deviates from the group track center, or from the land track center, the intensity distribution to the left and right of primary diffracted light diffracted at the edge of the track changes according to this offset. With a push-pull method, track offset is detected by utilizing this change in the intensity distribution. A TE signal obtained by push-pull method is called a push-pull TE signal (hereinafter referred to as a PPTE signal).
The PPTE signal that is the output signal from the subtracter 111 is inputted through the switch 115 to the tracking controller 116, is transmitted through a low frequency compensation circuit, a phase compensation circuit, or other such circuit made up of a digital filter involving a digital signal processor (hereinafter referred to as a DSP), and becomes a tracking drive signal. The tracking drive signal outputted from the tracking controller 116 goes through the adder 120 and amplified to a specific gain in the gain adjuster 121. The output signal from the gain adjuster 121 is inputted to and amplified by the tracking driver 122, and outputted to the tracking actuator 123.
The position of the objective lens 105 is controlled in the radial direction of the optical disk 506 by the above tracking control operation so that the optical beam focused on the optical disk 506 scans the desired track of the RAM region of the optical disk 506.
Next, the tracking control operation in the ROM region will be described.
The output signals from the preamps 109a and 109c are added by the adder 110a. The output signals from the preamps 109b and 109d are added by the adder 110b. The output signals from the adders 110a and 110b are converted into digitized signals by the comparators 112a and 112b, respectively. The digitized signals from the comparators 112a and 112b are compared for phase by the phase comparator 113, and pulses with a time width corresponding to the phase advance or phase delay of the edge are outputted. The pulse signals outputted from the phase comparator 113 are smoothed by the low pass filter 114 and become TE signals.
The above method for detecting TE signals is generally called a phase difference method. When the optical beam passes pits, the intensity distribution of the reflected light on the detector 108 varies with the position of the optical beam in the tracking direction, which produces a deviation in the phase of each of the diagonal sum signals of the four sub-regions. The phase difference method involves detecting track offset by utilizing this deviation in phase. A TE signal obtained by phase difference method will hereinafter be referred to as a phase difference TE signal.
The phase difference TE signal that is the output signal from the low pass filter 114 is inputted through the switch 115 to the tracking controller 116. The processing after this is the same as that in the tracking operation performed in the RAM region.
The position of the objective lens 105 is controlled in the radial direction of the optical disk 506 by the above tracking control operation so that the optical beam focused on the optical disk 506 scans the desired track of the ROM region of the optical disk 506.
The “search operation” will also be described through reference to FIG. 6. This search operation is an operation in which the optical beam is moved from a state of being located on a track in the RAM region to a state of being located on the desired track in the ROM region, or, conversely, an operation in which the optical beam is moved from the ROM region to the RAM region.
Before describing this “search operation,” the “address regeneration operation” will be described first. Address regeneration is an operation in which the current address of the light spot is obtained.
The output signals from the adders 110c and 110d are added by the adder 124, producing a signal corresponding to the total amount of light obtained at the detector 108. The output signal from the adder 124 (the total amount of light) is inputted to the address regenerator 125. The address regenerator 125 digitizes the input signal so as to read the address, and the read address is outputted to the microcomputer 119. The above address regeneration operation allows the optical disk device to obtain the current address of the light spot.
Next, the search operation from the RAM region to the ROM region will be described.
A boundary address ADb between the ROM region and the RAM region is stored in a memory 128. When the address ADt of the desired track is inputted to the microcomputer 119, the microcomputer 119 obtains the current address AD0 from the address regenerator 125 and calculates the number of tracks Nt (=AD0−ADt) between the current track and the desired track. The microcomputer 119 also compares the boundary address ADb with the desired track address ADt to find whether the desired track is in the ROM region, and calculates the number of tracks Nb (=AD0−ADb) until the ROM region is entered. The microcomputer 119 also produces a transport motor drive signal on the basis of the calculated number of tracks Nt, and outputs this signal to the transport motor driver 129. The transport motor driver 129 amplifies the transport motor drive signal and outputs it to the transport motor 130.
A PPTE signal is generated when the optical head 100 is moved by the transport motor 130 in the radial direction of the optical disk 506. This PPTE signal is inputted through the switch 115 to the comparator 126, where it is digitized. The pulse counter 127 counts the number of rising edges of the digitized signal from the comparator 126, so that the number of tracks Nc crossed by the optical beam since the start of the search operation is outputted to the microcomputer 119. The microcomputer 119 reads the number of tracks Nc crossed by the optical beam since the start of the search operation, and compares this number to see if Nc is greater or less than the number of tracks Nb until the ROM region is entered. If Nc is less than Nb, the microcomputer 119 leaves the output signal from the switch 115 as a PPTE signal. If Nc is greater than or equal to Nb, the microcomputer 119 switches the output signal from the switch 115 from a PPTE signal to a phase difference TE signal. Further, when the microcomputer 119 reads the number of tracks Nc crossed by the optical beam since the start of the search operation, if Nc is equal to Nt, the count of the pulse counter 127 is reset and tracking control is performed. The tracking control operation here is performed on the basis of the phase difference TE signal. After this, the microcomputer 119 obtains the current address from the address regenerator 125, and if the obtained address matches the desired address, the track search operation is concluded, but if there is no match, the above track search operation is repeated until the desired track is found.
The search operation from the ROM region to the RAM region is the same. Specifically, the microcomputer 119 compares the number of tracks Nb (=ADb−AD0) until the RAM region is entered to see if it is greater or less than the number of tracks Nc crossed by the optical beam since the start of the search operation. If Nc is less than Nb, the microcomputer 119 leaves the output signal from the switch 115 as a phase difference TE signal. If Nc is greater than or equal to Nb, the microcomputer 119 switches the output signal from the switch 115 from a phase difference TE signal to a PPTE signal. After this, if Nc and Nt are equal, the microcomputer 119 resets the count of the pulse counter 127, and performs tracking control. The tracking control operation here is performed on the basis of the PPTE signal.
As discussed above, with a conventional optical disk device, if the light spot is moved over the RAM region, a TE signal is produced by a PPTE signal detection method, and if the light spot is moving over the ROM region, a TE signal is produced by a phase difference TE signal detection method.
Further, this optical disk device is configured such that tracking control is performed by suitably switching the TE signal detection method according to the movement between the RAM region and the ROM region.
Another type of optical disk device is one that automatically adjusts the loop gain of tracking control in each region in order to ensure the control characteristics needed for the tracking control system (see, for example, Japanese Laid-Open Patent Application H4-19830 (pages 2 to 5, FIGS. 1 to 7)).
The adjustment of the loop gain in a tracking control system will now be described through reference to FIG. 6.
The microcomputer 119 generates a disturbance signal of a specific frequency by means of the built-in disturbance generator 118. This disturbance signal is applied to the tracking control system by the adder 120. Along with the generation and application of the disturbance signal, the microcomputer 119 also samples and takes in the response signal of the tracking control system with respect to this disturbance signal by means of the A/D converter 117. Further, the microcomputer 119 calculates the disturbance signal applied to the tracking control system and the response signal that is taken in, and measures the ratio between the applied disturbance signal and the incorporated response signal (hereinafter referred to as the loop gain), or the phase difference between the applied disturbance signal and the incorporated response signal (hereinafter referred to as the phase difference). After this, the microcomputer 119 actuates the gain adjuster 121 according to the measured loop gain or phase difference, so that the tracking control system is adjusted to a specific loop gain.
This loop gain adjustment operation results in the optimal loop gain for the tracking control system, allowing stable tracking control to be achieved.
As discussed above, this conventional optical disk device is configured such that loop gain adjustment results in the optimal loop gain for the tracking control system, allowing stable tracking control to be achieved. However, when the RAM region, which is composed of land and groove tracks, and the ROM region, which is composed of pit strings, are mixed together in a disk, as is the case with the optical disk 506, it is necessary to switch between two different detection methods for tracking control, which leads to a larger dedicated circuit in the optical disk device.
Furthermore, the loop gain adjustment has to be performed for the RAM region and for the ROM region in order to achieve stable tracking control in each region, so adjustment takes longer, which leads to lower performance of the optical disk device.
With the next-generation high-density optical disks that allow both recording and reproduction, the recording of information ahead of time in the ROM region is accomplished not by pit strings, but by minutely varying (wobbling) the track shape in the radial direction of the optical disk. In addition, the RAM region is formed by continuous, convex and concave guide grooves, just as with conventional configurations.
Employing the above configuration for these next-generation high-density optical disks allows the same TE signal detection method, namely, PPTE detection, to be used in both the ROM region and the RAM region.
When the recording of information in the ROM region of an optical disk is accomplished by the above-mentioned wobbling, the track pitch must be wider in the ROM region than in the RAM region in order to wobble the track. In other words, the optical disk has a structure in which the track pitch is different in the RAM region and the ROM region.
The following problems are encountered when a PPTE signal detection method is applied to an optical disk such as this that has a plurality of regions of different track pitch.
FIG. 8 is a diagram of the correspondence between the PPTE signal waveform and the tracks on an optical disk 106 having regions of different track pitch. FIG. 8a is a cross section of the optical disk 106 in its radial direction. As shown in the drawing, region 1 is a region with a track pitch of Tp1, while region 2 is a region of Tp2. FIG. 8b is a waveform diagram obtained by plotting the PPTE signal obtained at various locations along the horizontal axis in FIG. 8a. 
As shown in FIG. 8, the amplitude of the TE signal obtained by PPTE signal detection is dependent on the track pitch where the light spot is located. Accordingly, the detection sensitivity for the TE signal is different in regions 1 and 2, which have different track pitches. Specifically, even if the tracking control loop gain in region 1 is optimally adjusted by loop gain adjustment, the loop gain will still not be optimal in region 2. Therefore, a problem is that stable tracking control cannot be ensured in region 2 even when using an adjusted loop gain in region 1.
It is also possible to perform the adjustment of tracking control loop gain for each region in order to avoid this problem. In this case, the stability of tracking control can be ensured for all regions, but loop gain adjustment has to be carried out once for every region. Consequently, just as with a conventional optical disk device, this leads to longer adjustment time and lower performance of the optical disk device.
The present invention was conceived in an effort to solve the above problems, and provides an optical disk device that includes tracking control means for estimating the loop gain so that the desired tracking control characteristics will be obtained in all of the regions of an optical disk having a plurality of regions of different track pitch.